A Color Illustrated Guide To Constituents, Textures, Cements, and Porosities of Sandstones and Associated Rocks

Edited by Peter A. Scholle


This book is designed as a companion volume to AAPG Memoir 27. As with its predecessor volume, the purpose of this book is to provide identified illustrations of important grains, textures, cements, and porosity types for geologists who may not be specialists in the petrography of sandstones and associated sedimentary rocks.

Sandstone petrography is of particular interest to the explorationist for several reasons. First, it can provide valuable information on the detailed composition of sedimentary rocks. From this, one can often draw conclusions about the lithology, climate, and tectonic history of the source area, as well as predicting the response of such units to a variety of subsurface diagenetic environments. Second, one can acquire significant data on the grain size, sorting, and rounding of sedimentary grains. For Iithified sediments this may be the only way to obtain such data, which may be useful in determinations of the transport mechanisms and depositional environment of the sediment. Third, information may be obtained on the postdepositional alteration history of sedimentary rocks. This may include data on compaction, cementation, leaching, fracturing, porosity types, and other factors. These are essential for a proper understanding of reservoir rocks and, commonly, petrography provides the only technique forgathering accurate data on such diagenetic factors.

This book is intended as an introduction for exploration geologists or students and is by no meansa complete textbook or treatise. However, it does include a wide variety of color photographs of terrigenous clastic grains, cements, and textures of sandstones and common accessory rock types. Although most of the illustrations are of features seen with the petrographic microscope, some scanning electron micrographs are included. The illustrations were made from samples having as wide a range of lithologies, geologic ages, and localities as possible to insure a fairly representative presentation. In addition, the photographs were generally selected to show the most common grain and textural types encountered by the geologist and to present typical, rather than spectacular, examples of most features. Thus, the book shouId have applicability to any sandstone petrographic study.

This volume focuses on the descriptive aspects of petrography and includes no text other than figure captions. Bibliographies are provided in each section of the book. For more detailed descriptive and interpretive information, the references listed in both the general and specific bibliographies should be consulted.

The major emphasis of th is book is on the fou... major fabric elements of sandstones: framework grains; detrital fine-grained matrix; cements; and pore space.

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    Interparticle—very common.

    Intraparticle—rare, but possible within rock fragments, fossils, and other detrital grains.

    Intercrystal-rare? Remnant primary porosity can be significant within clay cements.


    Dissolution of detrital grains—common, especially removal of feldspars, carbonate rock fragments or fossils, or detrital sulphates.

    Dissolution of authigenic cements—very common removal of calcite, dolomite, and siderite; significant removal of gypsum and/or anhydrite.

    Dissolution of authigenic replacement minerals—common removal of carbonate or sulphate minerals.

    Shrinkage—minor; can be significant in glauconitic sediments.

    Fracturing—minor except locally.

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    Although petrography is an extremely valuable tool for the identification of minerals and their textural interrelations, it is best used (in many cases) in conjunction with other techniques.

    Precise mineral determination commonly is aided by staining of thin sections or rock slabs, by X-ray diffraction analysis, or by microprobe examination. Minerals present in small amounts may best be analyzed after separation and concentration using heavy liquids, shaker tables, or other techniques. Likewise, noncarbonate minerals in a carbonate host rock are normally better analyzed in acid-insoluble residues than in thin section. Where detailed understanding of the trace element chemistry of the sediments is essential, X-ray fluorescence, microprobe, atomic absorption, or cathodoluminescence techniques may be applicable.

    Commonly, sediments are too fine-grained for adequate examination with the light microscope. The practical limit of resolution of the best light microscopes is in the one to two micrometer (μm) range. Many detrital and authigenic grains such as clays, micritic carbonates, or organic matter fall within or below that size range. Furthermore, because most standard thin sections are about 30 μm thick, a researcher examines 10 to 20 of these small grains stacked on top of one another, with obvious loss of resolution. Smear mounts or grain mounts (slides with individual, disaggregated grains smeared or settled out onto the slide surface) are an aid in examining small grains where the material can be disaggregated into individual components. In most cases however, scanning and transmission electron microscopy are proved to be the most effective techniques for the detailed examination of fine-grained sediments.

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    Staining techniques are among the fastest, simplest, and cheapest methods for getting reliable data on composition of detrital grains or cements in sandstones. Stains for calcite, organic matter, K-feldspar, and plagioclase are among the most useful stains.

    The top photo shows a sandstone (Pennsylvanian Tensleep of Wyoming; 0.10 mm) which has been stained for calcite using Alizarin Red S. Calcite and dolomite are both present as cements but only the calcite has taken the red stain. Note imperfections in staining of calcite near thin edges of crystals and where bubbles were present. Because of the similarity in optical properties of calcite and dolomite, such staining is essential for accurate identification of these minerals.

    The lower photo shows a K-feldspar (sanidine) from a Tertiary intrusive in Nevada (0.28 mm). This originally colorless grain was stained for potassium using a sodium cobaltinitrite solution. The lack of twinning and cleavage in this grain make it difficult to differentiate from quartz without time-consuming optical study. Staining, however, provides a rapid and reliable alternative for routine petrographic studies.

    Directions for stain preparation are given in the references by Bailey and Stevens (1960), Laniz, et al (1964), Dickson (1966), Friedman (1971), and Whitlatch and Johnson (1974).

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    Cathodoluminescence can be an invaluable tool in petrographic studies. It provides information on the spatial distribution of trace elements in terrigenous clastic (as well as carbonate) grains and cements. Analysis can be done using polished rock chips, polished thin sections, or even unpolished and uncovered thin sections. The equipment requires costs about the same as a moderately priced polarizing microscope and can be installed on virtually any microscope.

    This example shows a sandstone from the Devonian Hoing Sandstone Member of the Cedar Valley limestone in Illinois. The upper photo, taken with transmitted light, shows a quartz arenite with elongate, sutured intergranular boundaries which might be considered as indicative of compaction and pressure solution. The lower photo, taken with cathodoluminescence, shows the same field of view with dramatically different results. The detrital grain cores, which luminesce orange and blue, can be seen to be well rounded, and touch each other only at point contacts. Subsequent quartz overgrowths (generally nonluminescent with some luminescent zones) have obliterated most porosity and give the appearance of a compacted fabric when cathodoluminescence is not used. The differences in luminescence between detrital grain cores and authigenic overgrowths is a function of subtle differences in their trace element composition.

    These differences are accentuated by long-exposure-time photography of small areas because of the inherent weakness of the luminescence. Photos by R. F. Sippel.

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    Fluid inclusions can be found in virtually all crystals. They range in size from less than 1 μm to a few centimeters, although inclusions larger than 1 mm are uncommon. Most contain a solution which represents a sample of the original waters from which the crystal formed, plus a gas or solid phase which may have separated during cooling.

    Careful petrographic study (commonly using heating or freezing stages) can determine the composition and original temperature of the fluids involved in crystal formation. This can provide useful information on the timing and conditions of cementation or mineralization, although care must be taken to determine the exact time relations of the fluid inclusions and the host mineral.

    The top photo shows two phase (fluid and liquid) inclusions from the fluorite-zinc district of Illinois (0.40 mm).

    The middle photo illustrates a three phase (solid, liquid, and gas) inclusion in a quartz geode from Iowa (10 μm).

    The bottom photo shows a two phase immiscible mixture of oil and water within a fluorite crystal from the same locality as the top photo. The colorless fluid is a strong brine; yellow fluid is oil; gas bubbles are methane associated with oil. All photos by Edwin Roedder.

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    Scanning electron microscopy provides two major advantages over light microscopy: an extreme depth of focus and a wide range of magnification. The top photo, from the Miocene ‘Hayner Ranch Formation’ of New Mexico (30 μm), illustrates the remarkable depth of focus of the SEM. Clinoptilolite crystals here fill pore space in a sandstone.

    The middle photo, from the Permian Rotliegendes Sandstone of the North Sea (7 μm), shows the excellent resolution of extremely small wispy terminations on authigenic illite cements.

    The bottom photo, also from the Rotliegendes Sandstone, illustrates a specialized technique—pore casting. The rock was pressure-impregnated with epoxy and the component grains were leached out subsequently with hydrofluoric acid. This leaves a three-dimensional network of epoxy which shows the geometry of the pore system, including small but interconnected pores not normally seen in thin section.

    Top photo by C. W. Keighin (courtesy of T. R. Walker); lower photos by E. D. Pittman.

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    Scanning electron microscopy is useful not only for examining sediment textures, but, when equipped with an energy dispersive analyser, it can be used effectively for mineral identification and semi-quantitative chemical analysis. The analyses are rapid (seconds) and require relatively little sample preparation. In most cases, small chips of the sample can be mounted on a small plug with no polishing or cutting required. The sample is then coated with a gold-palladium alloy (or other conductive metal) and is inserted into the SEM.

    Here, a potassium feldspar from the Cretaceous Frontier Formation of Wyoming is shown with an accompanying analytical spectrum. Although the grain might be identifiable as a feldspar on the basis of its crystal shape, cleavage, and other features alone, the energy dispersive analysis provides additional chemical data which allows positive identification.

    The analytical trace (lower photo) shows major peaks for Si and K (the main K peak has the long, pale blue line over it) with only very minor peaks for other elements. This indicates a rather pure K-feldspar composition.

    Although energy dispersive analysis on the SEM provides an excellent tool for mineral identification, it is not ideally suited for quantitative analytical work. Detailed determination of mineral composition or analysis of trace element contents of small crystals is best done using polished samples on an electron microprobe.

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    The electron microprobe, used in the sample current-image mode, is useful for study of textural relations of minerals. Contrast in the images produced is due to high-versus-low atomic number. On an electron microprobe equipped with a multichannel analyzer/energy dispersive detector system, mineral grains can be chemically characterized in a few seconds according to their spectra of elements.

    The four figures are electron microprobe sample current image photographs of granite in which the heavy minerals magnetite (mt), ilmenorutile (i), zircon (z), monazite (m). thorite (t), and fluocerite (fc) are surrounded by quartz and potassium feldspar (black).

    Microprobe sample preparation (polished sections) is relatively time consuming, and analytical work using the micro-probe is relatively complex and expensive. However, in many cases, microprobe analysis provides the only reliable method for obtaining quantitative data on major and minor element composition of minerals or accurate identification of extremely small grains. Photos by G. A. Desborough.

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